I agree that Mebujika is awesome but the problem is that both its sets have issues.
The full coverage set: Wood Horn/Return or Double Edge/Jump Kick/Wild Bolt has power issues, despite hitting everything powerfully it cant 2HKO a lot of things and Nattorei still shits on it. The SD set: SD/DE/Wood Horn/Jump Kick or Nature Power has more killing potential but loses out on a lot of its coverage meaning it gets walled by a lot more (eg Gyarados) and still gets shit on by Nattorei.
Regardless awesome poke.
Ferrothorn poses it most issues in that it can switch in on its STABs and cause a shitton of recoil damage to Mebu, as Jump Kick does 51.1-60.2% to standard Natt, and is a guaranteed 2HKO on more SpDef variants, or when factoring in SR. Likewise, Natt risks coming in on a SD and potentially being OHKOd by Jump Kick - the issue with this it the possibility of ghosts spoiling your fun, meaning prediction is required. Either way, Ferro itself doesn't truly shit on Mebu at all, especially not as much as a few other things do.
The main issue I have with the SD set is its vulnerability to priority, Burns, Toxic and recoil, making it imo much more useful as a fast revenger with 4 attacks, as other chlorophyll mons have much greater reliability as an outright sweeper (Venusaur I'm looking at you).
Personally I use Megahorn in the last slot to rid myself of Lati@s easily, as my team has few Gyarados problems. However very little actually walls a 4 attacks set, with Gengar at high health, Bronzong and Chandelure being pretty much it (Gyara if running Megahorn too). If you predict well most things can be 2HKOd.
He seems to me to be the Doryuuzu of Sun - frail and priority weak, with few true counters, but ones that will wall him if you attempt an early sweep, and excellent as a cleaner late game with SD (or sweeper if you kill said counters) or a revenge killer. Dory has an easier time with his resists and status immunities in getting the SD, however, and in that his attacks are more reliable and he is less crippled by contact recoil.